I see no reason at all that you can't leave the amaranth and millet out. Make up the difference with the chestnut and rice, or you could try another GF flour like tapioca or buckwheat. Barley is extremely low in gluten and is sometimes OK for folks with gluten-sensitivity.

I'd stay away from barley if cooking for friends or others who are gluten-free -- why take the chance? And if they are Celiacs, then you MUST stay away.

I'm glad the substitution worked. The amaranth and millet were likely there both to add a certain flavor (amaranth in particular is rather "earthy") and to jolt the protein level (rice flours are VERY low in protein, especially when compared to wheat flours, which is one of the reasons you can't -- successfully -- just replace wheat flour with rice).

Sometimes I find a recipe with small amounts of a flour I don't have on hand. If I have the whole grain/seed/nut, I use the small electric coffee grinder (aka my spice/flax grinder). I've done this for millet -- so I have an easily stored source rather than a more perishable flour.

The report! I had made three savory quick breads, each pretty different from the other and only one gluten free. And everyone liked the gluten-free one best. It was dominated by chestnut and white rice flour, with some tapioca and buckwheat flours (thanks again, Emily). The texture was great, the flavors--cheeses, herbs, nuts, olives, etc., worked too. Thanks!

I used this recipe from a blog by David Santori, which I found by googling something like gluten-free chestnut flour recipe:
http://frenchieandtheyankee...changing the flours as we talked about, leaving out the prosciutto (vegetarian friends), and adding some slivered almonds, because I didn't have as many pine nuts as I thought I had. Mine was much darker than what's shown for the original recipe, partly I guess because of my buckwheat flour, but I think my chestnut flour must be darker than his too.